Cédric Cleyrat

Frequent reduction or absence of detection of the JAK2-mutated clone in JAK2V617F-positive patients within the first years of hydroxyurea therapy

In this study, hydroxyurea treatment reduced the percentage of JAK2V617F mutant alleles by >30% in 13/25 patients with essential thrombocythemia or polycythemia vera within the first year after diagnosis. We analyzed the effect of hydroxyurea on the JAK2V617F allelic ratio (%JAK2V617F), measured in purified blood granulocytes, of patients with polycythemia vera and essential thrombocythemia. Thirty-six patients were examined sequentially prior to and after start of hydroxy-urea therapy (8 polycythemia vera, 17 essential thrombocythemia), or while remaining untreated (2 polycythemia vera, 9 essential thrombocythemia). Hydroxyurea therapy (median duration: 15 months) reduced the %JAK2V617F by >30% in 13/25 patients (4 polycythemia vera, 9 essential thrombocythemia). For 3 patients, JAK2V617F remained undetectable for 3–27 months. In addition, a single time point study of two large cohorts of patients, examined either at the time of diagnosis (99 polycythemia vera, 178 essential thrombocythemia) or while receiving hydroxyurea (36 polycythemia vera, 98 essential thrombocythemia; median length of therapy: 32 months), confirmed reduction of %JAK2V617F in the hydroxyurea-treated group (24% vs. 33% JAK2V617F at diagnosis, p<0.01). Prospective studies are needed to determine the prognostic value of reduced JAK2V617F allele burden under cytoreductive therapy.

Anti-inflammatory cytokines hepatocyte growth factor and interleukin-11 are over-expressed in Polycythemia vera and contribute to the growth of clonal erythroblasts independently of JAK2V617F

The V617F activating mutation of janus kinase 2 (JAK2), a kinase essential for cytokine signalling, characterizes Polycythemia vera (PV), one of the myeloproliferative neoplasms (MPN). However, not all MPNs carry mutations of JAK2, and in JAK2-mutated patients, expression of JAK2V617F does not always result in clone expansion. In the present study, we provide evidence that inflammation-linked cytokines are required for the growth of JAK2V617F-mutated erythroid progenitors. In a first series of experiments, we searched for cytokines over-expressed in PV using cytokine antibody (Ab) arrays, and enzyme-linked immunosorbent assays for analyses of serum and bone marrow (BM) plasma, and quantitative reverse transcription–PCRs for analyses of cells purified from PV patients and controls. We found that PV patients over-expressed anti-inflammatory hepatocyte growth factor (HGF) and interleukin-11 (IL-11), BM mesenchymal stromal cells (BMMSCs) and erythroblasts being the main producers. In a second series of experiments, autocrine/paracrine cytokine stimulation of erythroblasts was blocked using neutralizing Abs specific for IL-11 or c-MET, the HGF receptor. The growth of JAK2V617F-mutated HEL cells and PV erythroblasts was inhibited, indicating that JAK2-mutated cells depend on HGF and IL-11 for their growth. Additional experiments showed that transient expression of JAK2V617F in BaF-3/erythropoietin receptor cells, and invalidation of JAK2V617F in HEL cells using anti-JAK2 small interfering RNA, did not affect HGF and IL-11 expression. Thus, anti-inflammatory HGF and IL-11 are upregulated in PV and their overproduction is not a consequence of JAK2V617F. As both cytokines contribute to the proliferation of PV erythroblasts, blocking the c-MET/HGF/IL-11 pathways could be of interest as an additional therapeutic option in PV.

Mpl traffics to the cell surface through conventional and unconventional routes.

Myeloproliferative neoplasms (MPNs) are often characterized by JAK2 or calreticulin (CALR) mutations, indicating aberrant trafficking in pathogenesis. This study focuses on Mpl trafficking and Jak2 association using two model systems: human erythroleukemia cells (HEL; JAK2V617F) and K562 myeloid leukemia cells (JAK2WT). Consistent with a putative chaperone role for Jak2, Mpl and Jak2 associate on both intracellular and plasma membranes (shown by proximity ligation assay) and siRNA‐mediated knockdown of Jak2 led to Mpl trapping in the endoplasmic reticulum (ER). Even in Jak2 sufficient cells, Mpl accumulates in punctate structures that partially colocalize with ER‐tracker, the ER exit site marker (ERES) Sec31a, the autophagy marker LC3 and LAMP1. Mpl was fused to miniSOG, a genetically encoded tag for correlated light and electron microscopy. Results suggest that a fraction of Mpl is taken up into autophagic structures from the ER and routed to autolyososomes. Surface biotinylation shows that both immature and mature Mpl reach the cell surface; in K562 cells Mpl is also released in exosomes. Both forms rapidly internalize upon ligand addition, while recovery is primarily attributed to immature Mpl. Mpl appears to reach the plasma membrane via both conventional ER‐Golgi and autolysosome secretory pathways, as well as recycling.

Nested High-Resolution Melting Curve Analysis: A Highly Sensitive, Reliable, and Simple Method for Detection of Jak2 Exon 12 Mutations—Clinical Relevance in the Monitoring of Polycythemia

JAK2 exon 12 mutations are found in myeloproliferative disorders characterized by erythrocytosis. Lying in a 33-bp region and conserving the open reading frame, they often present a low allelic burden (<10%), which excludes screening with techniques such as allele-specific PCR or different sequencing protocols. High-resolution melting (HRM), a fast in-tube method, seems the most accurate routine technique for that. We describe a reliable and powerful nested HRM technique, independent of DNA preparation and with technical sensitivity of 100% (95% CI, 93% to 100%) and specificity of 96.7% (95% CI, 89.7% to 96.7%). Screening a cohort of 10 idiopathic erythrocytosis, 28 polycythemia vera, and 7 secondary erythrocytosis cases allowed the detection of 15 mutants, including 9 different mutations, of which 3 were unreported, all in the polycythemia vera group, and presented a characteristic profile: pure erythrocytosis associated with low serum erythropoietin. Threshold detection level ranged from 1% to 3% allelic burden, depending on the mutation. All of the HRM positive signals were found mutated by sequencing. Six of them (40%), however, required cloning before sequencing, because of low allelic burden. Classic techniques such as genomic sequencing may therefore miss patients with mutations. Given its sensitivity, HRM (and nested HRM) can be used in routine diagnosis and seems to be the most efficient of current techniques for detection of JAK2 exon 12 mutations.

The architectural relationship of components controlling mast cell endocytosis

Summary Eukaryotic cells use multiple routes for receptor internalization. Here, we examine the topographical relationships of clathrin-dependent and clathrin-independent endocytic structures on the plasma membranes of leukemia-derived mast cells. The high affinity IgE receptor (Fc&egr;RI) utilizes both pathways, whereas transferrin receptor serves as a marker for the classical clathrin-mediated endocytosis pathway. Both receptors were tracked by live-cell imaging in the presence or absence of inhibitors that established their differential dependence on specific endocytic adaptor proteins. The topology of antigen-bound Fc&egr;RI, clathrin, dynamin, Arf6 and Eps15-positive structures were analyzed by 2D and 3D immunoelectron microscopy techniques, revealing their remarkable spatial relationships and unique geometry. We conclude that the mast cell plasma membrane has multiple specialized domains for endocytosis. Their close proximity might reflect shared components, such as lipids and adaptor proteins, that facilitate inward membrane curvature. Intersections between these specialized domains might represent sorting stations that direct cargo to specific endocytic pathways.

Confocal imaging studies cast doubt on nuclear localization of JAK2V617F.

To the editor: The most common mutation in myeloproliferative neoplasm (MPN) substitutes a valine to phenylalanine at position 617 of JAK2 ( JAK2 V617F).[1][1]Recently, Rinaldi et al reported both a nuclear and cytoplasmic localization of JAK2 in K562 cells.[2][2] A previous report also indicated

Differential mast cell outcomes are sensitive to FcεRI-Syk binding kinetics

Single-molecule imaging was used to quantify the transient nature of FcεRI-Syk interactions in a rodent mast cell line. A functional mutation that increases Syk off-rate leads to altered Syk phosphorylation patterns and impaired signaling, highlighting the importance of finely tuned protein interactions in directing cellular outcomes.

Allergen Valency, Dose, and FcεRI Occupancy Set Thresholds for Secretory Responses to Pen a 1 and Motivate Design of Hypoallergens.

Ag-mediated crosslinking of IgE–FcεRI complexes activates mast cells and basophils, initiating the allergic response. Of 34 donors recruited having self-reported shrimp allergy, only 35% had significant levels of shrimp-specific IgE in serum and measurable basophil secretory responses to rPen a 1 (shrimp tropomyosin). We report that degranulation is linked to the number of FcεRI occupied with allergen-specific IgE, as well as the dose and valency of Pen a 1. Using clustered regularly interspaced palindromic repeat–based gene editing, human RBLrαKO cells were created that exclusively express the human FcεRIα subunit. Pen a 1–specific IgE was affinity purified from shrimp-positive plasma. Cells primed with a range of Pen a 1–specific IgE and challenged with Pen a 1 showed a bell-shaped dose response for secretion, with optimal Pen a 1 doses of 0.1–10 ng/ml. Mathematical modeling provided estimates of receptor aggregation kinetics based on FcεRI occupancy with IgE and allergen dose. Maximal degranulation was elicited when ∼2700 IgE–FcεRI complexes were occupied with specific IgE and challenged with Pen a 1 (IgE epitope valency of ≥8), although measurable responses were achieved when only a few hundred FcεRI were occupied. Prolonged periods of pepsin-mediated Pen a 1 proteolysis, which simulates gastric digestion, were required to diminish secretory responses. Recombinant fragments (60–79 aa), which together span the entire length of tropomyosin, were weak secretagogues. These fragments have reduced dimerization capacity, compete with intact Pen a 1 for binding to IgE–FcεRI complexes, and represent a starting point for the design of promising hypoallergens for immunotherapy.

Gene editing rescue of a novel MPL mutant associated with congenital amegakaryocytic thrombocytopenia

Thrombopoietin (Tpo) and its receptor (Mpl) are the principal regulators of early and late thrombopoiesis and hematopoietic stem cell maintenance. Mutations in MPL can drastically impair its function and be a contributing factor in multiple hematologic malignancies, including congenital amegakaryocytic thrombocytopenia (CAMT). CAMT is characterized by severe thrombocytopenia at birth, which progresses to bone marrow failure and pancytopenia. Here we report unique familial cases of CAMT that presented with a previously unreported MPL mutation: T814C (W272R) in the background of the activating MPL G117T (K39N or Baltimore) mutation. Confocal microscopy, proliferation and surface biotinylation assays, co-immunoprecipitation, and western blotting analysis were used to elucidate the function and trafficking of Mpl mutants. Results showed that Mpl protein bearing the W272R mutation, alone or together with the K39N mutation, lacks detectable surface expression while being strongly colocalized with the endoplasmic reticulum (ER) marker calreticulin. Both WT and K39N-mutated Mpl were found to be signaling competent, but single or double mutants bearing W272R were unresponsive to Tpo. Function of the deficient Mpl receptor could be rescued by using 2 separate approaches: (1) GRASP55 overexpression, which partially restored Tpo-induced signaling of mutant Mpl by activating an autophagy-dependent secretory pathway and thus forcing ER-trapped immature receptors to traffic to the cell surface; and (2) CRISPR-Cas9 gene editing used to repair MPL T814C mutation in transfected cell lines and primary umbilical cord blood-derived CD34+ cells. We demonstrate proof of principle for rescue of mutant Mpl function by using gene editing of primary hematopoietic stem cells, which indicates direct therapeutic applications for CAMT patients.

Note: Time-gated 3D single quantum dot tracking with simultaneous spinning disk imaging.

We describe recent upgrades to a 3D tracking microscope to include simultaneous Nipkow spinning disk imaging and time-gated single-particle tracking (SPT). Simultaneous 3D molecular tracking and spinning disk imaging enable the visualization of cellular structures and proteins around a given fluorescently labeled target molecule. The addition of photon time-gating to the SPT hardware improves signal to noise by discriminating against Raman scattering and short-lived fluorescence. In contrast to camera-based SPT, single-photon arrival times are recorded, enabling time-resolved spectroscopy (e.g., measurement of fluorescence lifetimes and photon correlations) to be performed during single molecule/particle tracking experiments.